Volume activated anion channel and astrocytic cellular edema in traumatic brain injury and stroke

Drug development in targeting ion channels for brain edema

Cerebral edema is a pathological hallmark of various central nervous system (CNS) insults, including traumatic brain injury (TBI) and excitotoxic injury such as stroke. Due to the rigidity of the skull, edema-induced increase of intracranial fluid significantly complicates severe CNS injuries by raising intracranial pressure and compromising perfusion. Mortality due to cerebral edema is high. With mortality rates up to 80% in severe cases of stroke, it is the leading cause of death within the first week. Similarly, cerebral edema is devastating for patients of TBI, accounting for up to 50% mortality. Currently, the available treatments for cerebral edema include hypothermia, osmotherapy, and surgery. However, these treatments only address the symptoms and often elicit adverse side effects, potentially in part due to non-specificity. There is an urgent need to identify effective pharmacological treatments for cerebral edema. Currently, ion channels represent the third-largest target class for drug development, but their roles in cerebral edema remain ill-defined. The present review aims to provide an overview of the proposed roles of ion channels and transporters (including aquaporins, SUR1-TRPM4, chloride channels, glucose transporters, and proton-sensitive channels) in mediating cerebral edema in acute ischemic stroke and TBI. We also focus on the pharmacological inhibitors for each target and potential therapeutic strategies that may be further pursued for the treatment of cerebral edema.

Coupled left-shift of Nav channels: modeling the Na⁺-loading and dysfunctional excitability of damaged axons

Injury to neural tissue renders voltage-gated Na⁺ (Nav) channels leaky. Even mild axonal trauma initiates Na⁺-loading, leading to secondary Ca²⁺-loading and white matter degeneration. The nodal isoform is Nav1.6 and for Nav1.6-expressing HEK-cells, traumatic whole cell stretch causes an immediate tetrodotoxin-sensitive Na⁺-leak. In stretch-damaged oocyte patches, Nav1.6 current undergoes damage-intensity dependent hyperpolarizing- (left-) shifts, but whether left-shift underlies injured-axon Nav-leak is uncertain. Nav1.6 inactivation (availability) is kinetically limited by (coupled to) Nav activation, yielding coupled left-shift (CLS) of the two processes: CLS should move the steady-state Nav1.6 "window conductance" closer to typical firing thresholds. Here we simulated excitability and ion homeostasis in free-running nodes of Ranvier to assess if hallmark injured-axon behaviors--Na⁺-loading, ectopic excitation, propagation block--would occur with Nav-CLS. Intact/traumatized axolemma ratios were varied, and for some simulations Na/K pumps were included, with varied in/outside volumes. We simulated saltatory propagation with one mid-axon node variously traumatized. While dissipating the [Na⁺] gradient and hyperactivating the Na/K pump, Nav-CLS generated neuropathic pain-like ectopic bursts. Depending on CLS magnitude, fraction of Nav channels affected, and pump intensity, tonic or burst firing or nodal inexcitability occurred, with [Na⁺] and [K⁺] fluctuating. Severe CLS-induced inexcitability did not preclude Na⁺-loading; in fact, the steady-state Na⁺-leaks elicited large pump currents. At a mid-axon node, mild CLS perturbed normal anterograde propagation, and severe CLS blocked saltatory propagation. These results suggest that in damaged excitable cells, Nav-CLS could initiate cellular deterioration with attendant hyper- or hypo-excitability. Healthy-cell versions of Nav-CLS, however, could contribute to physiological rhythmic firing.

The beta1 subunit of the Na,K-ATPase pump interacts with megalencephalic leucoencephalopathy with subcortical cysts protein 1 (MLC1) in brain astrocytes: new insights into MLC pathogenesis

Megalencephalic leucoencephalopathy with subcortical cysts (MLC) is a rare congenital leucodystrophy caused by mutations in MLC1, a membrane protein of unknown function. MLC1 expression in astrocyte end-feet contacting blood vessels and meninges, along with brain swelling, fluid cysts and myelin vacuolation observed in MLC patients, suggests a possible role for MLC1 in the regulation of fluid and ion homeostasis and cellular volume changes. To identify MLC1 direct interactors and dissect the molecular pathways in which MLC1 is involved, we used NH2-MLC1 domain as a bait to screen a human brain library in a yeast two-hybrid assay. We identified the β1 subunit of the Na,K-ATPase pump as one of the interacting clones and confirmed it by pull-downs, co-fractionation assays and immunofluorescence stainings in human and rat astrocytes in vitro and in brain tissue. By performing ouabain-affinity chromatography on astrocyte and brain extracts, we isolated MLC1 and the whole Na,K-ATPase enzyme in a multiprotein complex that included Kir4.1, syntrophin and dystrobrevin. Because Na,K-ATPase is involved in intracellular osmotic control and volume regulation, we investigated the effect of hypo-osmotic stress on MLC1/Na,K-ATPase relationship in astrocytes. We found that hypo-osmotic conditions increased MLC1 membrane expression and favoured MLC1/Na,K-ATPase-β1 association. Moreover, hypo-osmosis induced astrocyte swelling and the reversible formation of endosome-derived vacuoles, where the two proteins co-localized. These data suggest that through its interaction with Na,K-ATPase, MLC1 is involved in the control of intracellular osmotic conditions and volume regulation in astrocytes, opening new perspectives for understanding the pathological mechanisms of MLC disease.